a) Field of the Invention
The invention is directed to an arrangement and a method for metering target material for the generation of short-wavelength electromagnetic radiation from an energy beam induced plasma. It is applied in particular in EUV radiation sources for projection lithography in semiconductor chip fabrication.
b) Description to the Related Art
Reproducible mass-limited targets for pulsed energy input for plasma generation have gained acceptance, above all in radiation sources for projection lithography, because they minimize unwanted particle emission (debris) compared to other types of targets. An ideal mass-limited target is characterized in that the particle number in the focus of the energy beam is limited to the particles used for generating radiation.
Excess target material that is vaporized or sublimated or which, although ionized, is not excited by the energy beam to a sufficient degree for the desired radiation emission (marginal area or immediate surroundings of the interaction point) causes not only increased emission of debris but also an unwanted gas atmosphere in the interaction chamber which in turn contributes considerably to an absorption of the short-wavelength EUV radiation generated from the plasma.
There are a number of embodiment forms of mass-limited targets known from the prior art. These are listed in the following along with their characteristic disadvantages:                Continuous liquid jet, possibly also frozen (solid consistency) (EP 0 895 706 B1)                    Mass limiting can be realized only to a limited extent because of the large size of the target in one linear dimension, resulting in increased debris and an unwanted gas burden in the vacuum chamber.            The shock wave proceeding from the plasma expansion (with slight damping) in the target jet in the direction of the target nozzle leads to a certain destruction of the target flow and, therefore, to a limiting of the pulse repetition rate of the laser excitation.                        Clusters (U.S. Pat. No. 5,577,092), gas puffs (Fiedorowicz et al., SPIE Proceedings, Vol. 4688, 619) and aerosols (WO 01/30122 A1; U.S. Pat. No. 6,324,256 B1)                    lead to severe nozzle erosion with short distances between the interaction point and the target nozzle and, at large distances from the nozzle (due to dramatically decreasing average density of the target), to a low efficiency of the radiation emission of the plasma.                        Continuous flow of individual droplets (EP 0 186 491 B1)                    requires precise synchronization with the excitation laser,            cold target material in the vicinity of the plasma (less than with the target jet, but still present) is vaporized and leads to absorbent gas atmosphere and increased debris.                        
All of the so-called mass-limited targets mentioned above have in common that there is more target material in the interaction chamber than is needed for generating the emitting plasma in spite of limiting the diameter of the target flow. With a continuous flow of droplets, for example, only about every hundredth drop is struck by the laser pulse. Apart from increased generation of debris, this leads to excess target material in the interaction chamber which causes an increased gas burden (particularly when xenon is used as target) and, therefore, an increased pressure in the interaction chamber. The increased gas burden leads in turn to an unwanted increase in the absorption of radiation emitted by the plasma. Further, the unused target material leads to increased material consumption and accordingly raises costs unnecessarily.